The present invention relates to processes for producing and recovering an organic acid, such as lactic acid, and esters thereof.
Organic acids such as lactic acid have a number of commercial uses, for example in food manufacturing, pharmaceuticals, plastics, textiles, and as a starting material in various chemical processes. It is well known to produce organic acids by fermentation of sugars, starch, or cheese whey, using microorganisms such as Lactobacillus delbrueckii to convert monosaccharides such as glucose, fructose, or galactose, or disaccharides such as sucrose or lactose, into organic acids such as lactic acid. The broth that results from fermentation contains unfermented sugars, carbohydrates, amino acids, proteins, and salts, as well as the acid. Some of these materials cause an undesirable color. The acid usually therefore must be recovered from the fermentation broth before it can be put to any substantial use.
During the production of an organic acid such as lactic acid by fermentation, the increasing concentration of the acid in the fermentation broth reduces the pH. As the pH decreases, the growth of the microorganism is inhibited and eventually stops, and therefore acid production stops. To prevent this, the pH of the fermentation broth typically is controlled by adding a base for neutralization, such as ammonia or a sodium or calcium base. However, one result of the addition of such a base is the formation of a salt of the acid (e.g., ammonium lactate). Therefore, it is often necessary to convert the salt to free acid or another form such as an ester, which subsequently can be converted to the free acid. The formation of the free acid we term here "acidification". The production of the ester can be advantageous as the ester can be distilled to produce a product of very high purity, which can then subsequently be converted back to a free acid of high purity. This serves to meet another of the process objectives, which we term "purification".
Many different processes have been suggested to convert the salt to free acid, including addition of a strong acid such as sulfuric acid followed by precipitation of the formed salt such as calcium sulfate; ion exchange; electrodialysis; and other methods. Many of these methods have the disadvantage of producing large quantities of waste or by-product such as calcium sulfate.
Other methods for conversion of the salt of the acid into free acid tend to be expensive. Electrodialysis for conversion of salts of organic acids into free organic acid and free base solution is expensive in both capital and operating costs. Another method that has been proposed involves the use of high pressure carbon dioxide in the presence of an amine extractant. This is cheaper than electrodialysis but is rather complex and may be difficult to operate.
Lactic acid is one organic acid of particular interest today because of a great projected demand for use as a polymer feedstock. As mentioned above, production of lactic acid by fermentation is most efficient at a pH range where the lactic acid is largely present as a salt. Thus recovery of pure lactic acid requires conversion of the salt into free acid. Purification steps are also required. One such method is production of a lactate ester and purification of the ester. Esters are organic chemicals of significant industrial importance, for example for use as solvents and as reagents. One way to form esters is by reacting an organic acid with an alcohol to form an ester and water, as shown in reaction (1): EQU R.sub.1 --COOH+R.sub.2 --CH.sub.2 OH.revreaction.R.sub.1 --COO--CH.sub.2 R.sub.2 +H.sub.2 O EQU organic acid+alcohol.revreaction.ester+water Reaction (1)
Many esterification processes or systems focus on removal of water to drive the yield or conversion. Removal of water biases the equilibrium towards the products shown on the right hand side of Equation (1). This approach to esterification has been successfully applied to a wide range of organic acids and alcohols.
Ammonium salts of organic acids can be esterified as follows: EQU R.sub.1 --COO.sup.- NH.sub.4.sup.++ R.sub.2 --CH.sub.2 H.revreaction.R.sub.1 --COO--CH.sub.2 R.sub.2 +NH.sub.3 +H.sub.2 O EQU ammonium salt of organic acid+alcohol.revreaction.ester+ammonia+water Reaction (2)
In this process, the ammonia and the water are ideally removed from the reaction medium to enhance the rate of reaction.
However, a reaction that is known to proceed quite readily is the following: EQU R.sub.1 -COOH+NH.sub.3.revreaction.R.sub.1 --CONH.sub.2 +H.sub.2 O EQU organic acid+ammonia.revreaction.amide+water Reaction (3)
Another reaction that can occur is:
ester+ammonia.revreaction.amide+alcohol Reaction (4)
This is usually considered an unwanted side reaction.
Another reaction that can take place involves alcoholysis of an amide, which can be acid catalyzed: EQU amide+alcohol.revreaction.ester+ammonia Reaction (5)
This reaction tends to proceed rather slowly and is not expected to be part of reaction (2). However, it can be a useful side reaction.
Another reaction that may occur is: EQU ammonia+acid.revreaction.ammonium salt Reaction (6)
This reaction may occur as part of reaction (2).
The amide is usually an undesirable product in organic acid manufacturing. Prior attempts to make reaction (2) proceed successfully have been restricted in yield by the tendency for reaction (3) to occur at the same time. Additionally, in normal operation in systems at ambient temperature reaction (2) proceeds relatively slowly.
A number of researchers have attempted to develop methods for conversion of ammonium salts into free acids or esters.
Filachione and Costello (Industrial Engineering and Chemistry Volume 44, Page 2189, 1952) describe a method for esterification of ammonium lactate directly with butanol or other alcohols with four or more carbon atoms. For example, an aqueous ammonium lactate solution and n-butanol were charged to a reactor vessel and refluxed for 4 to 8 hours at 105 to 145.degree. C. Typically during the course of the batch run the temperature rose, as water was slowly removed, driving the reaction towards the n-butyl lactate product. Ammonia and water products were taken off overhead together with the n-butanol--water azeotrope. The heterogeneous azeotroping agent was condensed, the alcohol phase returned to the reactor, and the water phase with dissolved ammonia was removed. The process achieved from 61 to 92% ammonia removal and from 49 to 67% conversion to butyl lactate. When the process used residue from previous steps, only part of the residue was recovered. An unwanted, yield reducing by-product builds up in the system. Filachione et al (U.S. Pat. No. 2,565,487, Aug. 28, 1951) also describe this direct esterification process.
It should be noted that lower alcohols can be used for esterification if benzene or some other azeotroping agent is added to the mixture. In these cases, the benzene-water forms the overhead heterogeneous azeotrope that is used to remove water to drive the reaction.
This process is not economical or practical due to the buildup of unwanted side reaction products, the long reaction times, and the relatively poor yields. For the case with ethanol, the added heterogeneous azeotroping agent adds to process complexity and safety concerns.
Schulz et al (U.S. Pat. No. 2,722,541, Nov. 1, 1955) describe equipment and a process to react ammonium lactate with butanol to make butyl lactate. They use several countercurrent reactors in series rather than the single reactor used by Filachione et al. However, they do not achieve significantly better yields or rates than Filachione et al.
Mercier (U.S. Pat. No. 4,100,189, Jul. 11, 1978) describes a process for recovery of free acetic acid. This process commences with the extraction of free acetic acid into a solvent, butyl-acetate mixed with n-butanol, which also will later act as an azeotroping agent for water removal. After the initial solvent extraction, Mercier back extracts the free acetic acid into ammonia and generates an aqueous ammonium acetate solution. This solution is then thermally decomposed to give ammonia and acetic acid in part of a complex system of columns and recycles. The decomposition temperature is 90 to 130.degree. C. The formation of esters and the formation of side reaction products is not addressed in this patent.
Walkup et al (U.S. Pat. No. 5,071,754, Dec. 10, 1991, and U.S. Pat. No. 5,252,473, Oct. 12, 1993) propose a process wherein ammonium lactate is reacted with alcohol in presence of stoichiometric excess gaseous carbon dioxide. An overhead gas stream containing carbon dioxide and ammonia is drawn off. A heavy bottoms stream is produced that contains the lactate ester. This is very similar to the direct esterification of ammonium lactate, as reported by Filachione et al. The added carbon dioxide acts to reduce the reaction time by providing an acidic material to help produce free lactic acid which can react with the alcohol present. The reaction time is reduced from 10 hours to 1 hour. However, yield remains about 70% conversion overall to the ester. The Walkup process is preferably operated at 160-180.degree. C. reaction temperature, with a 10:1 to 1:1 alcohol:acid ratio, and with a pressure of carbon dioxide of from 1 to 200 times atmospheric pressure.
Sterzel et al (U.S. Pat. No. 5,453,365, Sept. 26, 1995) describe a multi-step process for conversion of salts of organic acids to esters. The first step involves addition of calcium bicarbonate or similar species to a lactic acid producing fermentation to control the pH and form a crude aqueous solution of calcium salt of the lactic acid. In the second step, ammonia and carbon dioxide are added to the broth to reach a pH of 7 to 13 and to precipitate calcium carbonate. This calcium salt is removed by filtration or centrifugation for reuse in the fermentation. The crude ammonium lactate solution is then directly esterified with an alcohol. However this process gave significant levels of unwanted yield-reducing products such as lactamide. Therefore, an additional step was added to the process. This additional step involved displacing the ammonia with a trialkyl-amine. The trialkyl-amine does not react with the lactic acid to form amides. The ammonia is removed and then the solution containing the low molecular weight trialkyl-amine and the lactic acid is reacted with alcohol to produce ester.
The Sterzel patent indicates that the yield-reducing side reactions encountered by Filachione, Schulz, Walkup and Sterzel can be avoided by converting the ammonium lactate to the trialkyl-amine--lactate prior to esterification and also prior to the acid-base separation step. Sterzel et al then remove the base from the lactate by thermal decomposition wherein the low molecular weight trialkyl-amine is boiled overhead during the esterification. The esterification overhead contains four components--the ester, excess alcohol, water, and the trialkyl-amine.
Datta et al (U.S. Pat. No. 5,723,639, Mar. 3, 1998) describe use of pervaporation membranes for esterification to remove water. They also mention direct esterification of ammonium lactate with ethanol. However, the rates of pervaporation are relatively low, the cost of membranes is high, and the temperature required for ammonium lactate decomposition is greater than that for which membranes are currently available.
Two questions that are not addressed effectively in the above patents are (1) how to drive the removal of ammonia and (2) how to drive the esterification reaction to high levels of conversion, while in both cases minimizing the formation of unwanted side reaction products, and how to achieve this at high rates for either or both processes with relatively simple equipment. There is a long standing need for improved processes for producing and recovering organic acids and esters thereof.